Evapotranspiration (ET) is the key water flux at the interface of soil, vegetation and atmosphere. Methods to derive this flux or its individual components from in-situ measurements have been developed in various research disciplines, covering different scales from e.g. point scale sap flow or soil heat pulse measurements, via pedon-scale of lysimeters, ecosystem scale of eddy covariance footprints to the landscape scale via drones or scintillometers. In-situ-measurements are necessary for calibration, validation and comparisons with larger scale estimates from remote sensing and modelling, but scaling procedures and uncertainty estimations are required for meaningful comparisons. Additionally, the support of these processes by AI methods holds much promise, but usually depends on large well-described data sets.

This session will mainly focus on the variety of in-situ ET estimates such as from sap flow or soil heat pulse sensors, lysimeters, eddy covariance stations, scintillometers and other (possibly new) methods. We would also like to address the challenges in comparing the different in-situ estimates while dealing with scale-dependency, uncertainty and representativity. We welcome contributions that (1) assess and compare established and new in-situ measurements, (2) address error sources and uncertainty considerations of the respective methods, (3) bridge scales between different in-situ measurements and modelled and remotely sensed ET, (4) evaluate challenges and opportunities of using AI for in-situ scaling and comparisons.